1. Field of the Invention
The invention relates to an electromagnetic frictional engagement apparatus that causes a frictional force to be generated between a plurality of soft magnetic bodies by a magnetic force.
2. Description of the Related Art
Clutch apparatuses that cause a frictional force to be generated between a plurality of soft magnetic bodies by a magnetic force have been used, for example, for vehicles.
A driving force transmitting apparatus described in Japanese Patent Application Publication No. 2005-3167 (JP 2005-3167 A) includes an electromagnetic frictional engagement apparatus including an electromagnetic pilot clutch and a main clutch. The main clutch has a cam mechanism that converts a torque transmitted by the pilot clutch into an axial thrust, and a main outer clutch plate and a main inner clutch plate that are pressed by the thrust from the cam mechanism. The pilot clutch and the main clutch are disposed in a housing space in a bottomed cylindrical housing. A lubricant is sealed in the housing space.
The pilot clutch includes a pilot outer clutch plate and a pilot inner clutch plate that are soft magnetic bodies, and is disposed between a rear housing and an armature that are magnetic-path forming members. An electromagnetic coil and a yoke are disposed in a recessed portion formed in the rear housing. When a magnetic force of the electromagnetic coil attracts the armature toward the rear housing, the pilot outer clutch plate and the pilot inner clutch plate come into frictional contact with each other. A magnetic flux resulting from conduction of a current through the electromagnetic coil passes through the yoke, the rear housing, the pilot outer clutch plate, the pilot inner clutch plate, and the armature. A air gap is formed between the yoke and the rear housing and is contained in the magnetic path of the magnetic flux resulting from conduction of a current through the electromagnetic coil.
In the pilot outer clutch plate, lubrication grooves are formed through which a lubricant flows. The lubricant flowing through the lubrication grooves suppresses friction when the pilot outer clutch plate and the pilot inner clutch plate rotate relatively while frictioning each other, and also inhibits possible seizure of the pilot clutch.
The electromagnetic clutch (pilot clutch) configured as described above may be subjected to a slight torque transmitted even in an inoperative state where conduction of a current through the electromagnetic coil is blocked, that is, what is called a drag torque. The drag torque results from the viscosity of the lubricant when the distance between the relatively rotating members is short.
The inventors of the present invention have made earnest effort to reduce the drag torque and found that the drag torque can be reduced by increasing the width or the number of lubrication grooves to make the groove area of the lubrication grooves larger to allow a sufficient amount of lubricant to be fed to between the relatively rotating members. However, the increased groove area may increase resistance of a magnetic circuit to reduce the frictional force that can be generated between these members.